Case Study Paper
Antibiotics in the Environment
Over the last fifty years, public awareness of the long-term effects of chemicals
and pesticides has increased due to the anticipation of adverse human and ecological
health effects. Industrial activities, and their resulting by-products in wastewater, have
long been studied and regulated by the government for this reason. Research has shown
that many chemicals manufactured and used today enter the environment, disperse, and
persist for much longer than originally expected (Koplin 2002). However, little research
is available on the effects that humans, in everyday life and activities, are having on the
Household chemicals (e.g. detergents, deodorizers, degreasers), pharmaceuticals
(e.g. hormones, steroids, antibiotics), and other personal care products (PCP’s) (e.g.
antacids, caffeine, fragrances) are being washed down sinks and flushed down toilets all
over the world without a second thought. Most of these chemicals are not regulated in
any way and their potential health effects and acute toxicities in the environment are not
known (Halling-Sorensen et al. 1997). In the past it was thought that the key to dealing
with this type of waste was merely to dilute it by releasing the contaminated water into
streams, rivers, or out to sea. However, as the human population continues to rise so to
does our dependence on the Earth’s limited freshwater supplies. With more contaminants
being released into this resource every year, the world has started to think about the long-
term effects of this action.
Antibiotic usage has received a lot of attention in the media in the last several
years due to the increasing numbers of diseases becoming resistant to traditional
treatments. According to the Center for Disease Control (CDC), approximately 70
percent of infections that people get while hospitalized are now resistant to at least one
antibiotic (Ephraim 1999). Antibiotics are not entirely processed by our bodies when we
take them for medical purposes. Some are expelled as waste and wind up in wastewater
treatment plants where even the best tertiary treatment does little to disable antibiotic
activity (World Health Organization 1997). This wastewater is then released into our
waterways where it gets transported to larger areas. Microbial populations in the water
and sediments change when exposed to antibiotics and antibacterial agents. In some
cases some trophic levels may be completely wiped out causing community structure to
be remarkably changed (Wollenberger et al. 2000). This effect can make its way up the
food chain and may be a cause of the trends we are now seeing towards lower
Sources of Contamination
Sources of antibiotic contamination in our environment are more than just
consumers expelling unabsorbed medications through excretion into septic systems and
wastewater treatment plants. Effluent from pharmaceutical manufacturing plants
contains antibiotics. Landfills, though considered to be contained, can also be sources.
Sewage and wastewater from hospitals and veterinary clinics are also huge contributors
to this problem (Rhodes et al. 2000). Some of the largest sources of antibiotics in the
waterways are animal farms, crop production, and fish farms (Wiggins et al. 1999). In
animal production, antibiotics are commonly used at subtherapeutic levels in animal
feeds as growth promoters. They are also added to fishery waters as growth promoters or
as preventative maintenance.
Pathways into the Environment
The routes of antibiotic introduction to water are wide ranging. Antibiotics are
directly introduced into surface waters when fisheries use medicated foods or treat for
disease outbreaks. About 24 million pounds of antibiotics are fed to farmed animals
every year (Halling-Sorensen et al. 1998). Pathways created by animal farming range
from waste run-off, to manure being used as fertilizer, to other animals (e.g. birds,
rodents) eating or transporting the treated food.
About 300,000 pounds of antibiotics are used in crop production each year
(Halling-Sorensen et al. 1998). They are sprayed on high-value crops such as fruit trees
to prevent bacterial infections. Not all spray remains on the fruit; most of the antibiotics
are washed into the soil and eventually can be transported to surface or groundwaters.
Effluent from water treatment facilities is deposited directly into surface water at
outfall stations. Leachate from septic systems and landfills is released into the
unsaturated zone, but depending on soil conditions it may seep into groundwater or
spread laterally until it meets a stream or other surface water.
There has not been a lot of published research about the transport processes of
antibiotics. The mobility of antibiotics is anticipated to be similar to that of pesticides
because many possess the same physio-chemical properties (Halling-Sorensen et al.
1998). This suggests that antibiotic mobility can be modeled after known pesticide
mobility models. From pesticide research, it is well known that after application,
pesticides are capable of seeping into the ground to be transported into groundwater or
surface waters (Jones et al. 2001).
Fate, Degradation Pathways and Persistence of Antibiotics in the Environment
The environmental fate and degradation of antibiotics in waterways has been
investigated much more in Europe and Canada than in the United States. Sorption and
mobility studies may give an indication of the potential for biodegradation or persistence
of antibiotics in the environment. Substances with high sorption to minerals or organic
material in soils or manure are likely to have slow degradation rates, as they are
unavailable for degradation by microorganisms (Jensen 2001). Unfortunately, research
has shown that the physio-chemical characteristics of individual antibiotics does not
always correlate with their affinity for sorption. Oxytetracycline (OTC), a commonly
used antibiotic in both terrestrial and aquatic animal farming, has a Kd value of more than
1000 making it highly immobile in soil. However, the antibiotics metronidazole and
olanquindox, which are used interchangeably with OTC in farming activities, are fully
recoverable in leachate (Raboelle and Spliid 2000).
Antibiotics which have an affinity for absorbing onto particulate matter,
especially in the marine environment, are kept from being distributed by water
movement, but may persist and remain active for much longer in the environment
(Halling-Sorensen et al. 1998). In sediments retrieved under fish farming activities OTC
was found to be capable of causing antimicrobal effects up to 12 weeks after
administration in surface sediments (Jacobsen and Berglind 1988 IN Halling Sorensen et
al. 1998). Anoxic sediments are common in the aquatic environment and most antibiotic
compounds persist much longer in these anoxic conditions. Hektoen et al. (1995)
showed that antibiotics buried in sediments as shallow as 1-7 cm can have half-lives of
more than 300 days. This means that antibiotics can build up in the aquatic environment
to dangerous levels that may effect benthic communities and continue up through the
In aerobic soils many antibiotics used in agriculture degrade relatively quickly,
with half-lives ranging from 22-80 days, into non-degradable metabolites (e.g. ceftiofur
sodium, monccin and sarafloxacin hydrochloride) (Velagaleti et al. 1984 IN Daughton
and Jones-Lepp 2001). Soil sterilization inhibits the degradation of these substances
which has led researchers to believe that micro-organisms may be responsible for their
breakdown (Jensen 2001). Still, other antibiotics respond differently to breakdown.
Researchers have found that drug metabolites of chlorotetracycline excreted by
medicated livestock (e.g. as glucuronides) are decomposed by bacterial action in liquid
manure and reconverted into active drugs (Warman and Thomas 1981).
Many antibiotics used by humans do not biodegrade when passed through
traditional sewage treatment facilities; these include tetracycline, and most sulfa-based
antibiotics (Richardson and Bowron 1985). Once released into the waterways some
biodegradation takes place depending in the antibiotic (Table 1).
Antibiotic Use Degradation Fate/ Persistence
Erythromycin Growth Biodegradation T1/2 = 11.5days
promoter 97% active after 30 days
Neomycin Antibiotic Excretion 97% excreted in feces
after oral dosage
Oxolinic Acid Feed additive in Biodegradation T1/2 = 150-1000 days
fish farming depending on depth
Oxytetracycline Feed additive in Biodegradation T1/2 = 9 to 419 days
fish farming under anoxic conditions
Sulphatrimetroprim Antibiotic Biodegradation Within 1 year 75%
undegraded in surface
Tylosin Growth Biodegradation Temperature dependant,
Promoter temp. above 20oC
Usage, fate and degradation pathway for selected common antibiotics used medicinally.
Adapted from: Halling-Sorensen et al. 1998
Effects on the Environment
The results of antibiotics entering our waterways are not widely known; this is
because it has only been over the last few years that people have started to become
concerned about the potential effects. Another reason is that the concentrations of
antibiotics found in waters are usually quite low, in the low parts per billion range, and
there have not been reliable analytical methods to measure these low concentrations
(Koplin et al. 2002). Though individual antibiotic concentrations are low, there are so
many different antibiotics that when combined they could lead to serious health and
environmental problems. Little is known about the potential interactive effects that may
occur from these complex mixtures, let alone the metabolites that can be formed as they
break down (Koplin et al. 2002).
Some of the major concerns are that entire trophic levels of bacteria will be wiped
out in some ecosystems or that multiple drug resistant bacteria will flourish and make its
way into the food chain. Unfortunately, both of these concerns have been realized.
Effects on Biota
When evaluating the effects of antibiotics on microbial communities it is
important to keep in mind that target organisms vary between antibiotics. Antibiotics
may have a broad spectrum of activity or be active against one family of bacteria (e.g.
gram-negative or gram-positive). Indigenous communities of bacterial and fungal
populations are very complex and they have the important task of cycling nutrients.
Some processes are driven by just a few species, where others, such as decomposition of
organic matter, are driven by teamwork between many types of microorganisms.
Proper cycling of nutrients is critical for quality soils and essential for maintaining
sustainable use of agricultural lands. Nitrogen is one of the most important nutrients for
agricultural systems, and its cycling is driven by only two genera of gram-negative
bacteria (e.g. Nitrosomonas and Nitrobacter) (Jensen 2001). Gram-negative and wide
spectrum antibiotics, such as sulfonamides and tetracyclines could seriously inhibit
nutrient cycling if concentrations reached high enough levels. This result has been
observed in laboratory studies, but no field studies have found antibiotic concentrations at
levels that would seriously disrupt the nitrification process (Jensen 2001).
Oxolinic acid, which is commonly used in the fish farming industry, has been
shown in laboratory experiments to be extremely toxic to Daphnia magna, a common
freshwater crustacean; reproductive abilities were completely destroyed by levels of this
antibiotic at one order of magnitude lower than acute toxic levels (Wollenberger et al.
2000). This may result in serious disruption of trophic levels in these areas since
Daphnia are a major food source in freshwater systems.
Calaniod copepods (Temora turbinata) have been shown to have decreased adult
size, abnormal growth patterns and reduced egg production when exposed to OTC at
concentrations above 1ppm (Halling-Sorensen et al. 1997). OTC also severely affects
plants; Studies done with pinto beans (Phaseolus vulgaris) showed significant depression
of dry weights and root structure when watered with a solution of 10 mg kg -1 (Batchelder
It is unlikely that concentrations of antibiotics would be found in high enough
concentrations on farms where manure from medicated animals was spread to disrupt
bacterial colonies. However, if bacterial populations were altered as the result of
antibiotic contamination the feeding of microbivore species like mites and nematodes,
who are strongly linked to their bacterial food source, would be significantly impacted
and this trend could continue up the food chain (Beare et al. 1992).
Many bacterial strains tend to accumulate in high densities in biofilms on water
surfaces (Schwartz et al. 2002). When bacteria, even those in different taxonomic
affiliations, are in close contact with each other they have the ability to transfer genes
between them that resist antibiotics (Davidson 1999). Biofilms from wastewater
systems, streams, and even drinking water have been shown to breed antibiotic resistant
bacteria at a much higher rate than basic bulk water (Schwartz et al. 2002). In streams,
the extent of dispersion of antibiotic resistant bacteria is limited only by stream flow and
settling rates (Leff et al. 1998). Rivers contaminated with urban effluent and agricultural
runoff have been shown to have greater antibiotic resistant bacteria populations than
areas upstream of the contamination source (McArthur and Tuckfield 2000, Wiggins et
al. 1999). Researchers examined antibiotic resistance of natural bacteria communities in a
highly industrialized stream and a natural stream. They found statistically significant
patterns of resistance in bacteria that increased with proximity to industrial wastewater
outfalls. They were also able to positively correlate this resistance with mercury
concentrations in sediments. The authors imply that heavy metal pollution may
contribute to antibiotic resistance (McArthur and Tuckfield 2000).
The practice of feeding subtherapeutic levels of antibiotics to animals has led to
drug resistant bacteria infections, such as Salmonella typimurium, Escherichia coli and
Enterococcus, increasing clinically as animal antibiotic use has risen (Jones et al. 2001).
Due to the application of manure from medicated livestock being applied to agricultural
soils, multiple drug resistance has developed in the micro-flora and intestinal flora of
livestock and even untreated pigs (Halling-Sorenesen et al. 1998).
Fish farms are also targeted as producers of antibiotic resistant bacteria. The use
of OTC in aquaculture has been shown to cause a seasonal shift in bacterial species
towards Enterobacteriaceae and is associated with antibiotic resistance (Wollenberger et
al. 2000, Guardabassi et al. 1999). Samples taken from gills and intestines of wild
commercial fish captured near fish farming activities have shown high frequencies of
multiple antibiotic resistance (Rhodes et al. 2000, Guardabassi et al. 1999).
Currently there are no regulations for the monitoring of any antibiotics in ground,
surface, or drinking waters. This is because concentrations of antibiotics are generally
low, in the parts per billion range and are deemed by the Environmental Risk Assessment
(ERA) to have no significant effect on the environment (Velagaleti and Gill 2001).
Regulations that are in effect now relate to the disposal of unused or expired antibiotics
under the Current Good Manufacturing Practice (CGMP) regulations set forth by the
FDA (FDA 1998). This regulation calls for the incineration of all disposed of antibiotics
by the manufacturer (FDA 1998). However, with more awareness of the effects of this
type of pollution, the scientific community is beginning to recognize the importance of
structuring plans to begin regulating as the need arises. The American Chemical Society
holds yearly symposiums based on current scientific research; last years topic was:
Pharmaceuticals and personal care products in the environment: scientific and regulatory
issues. This topic is going to be a hot issue for years to come.
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